Biaxially oriented films made of synthetic material are obtained from an extruded strip once the material has been deposited in the molten state on a casting drum. The strip then passes into a longitudinal drawing machine in which drawing is accomplished due to the speed differential between various successive cylinders. The strip then passes into a transverse drawing machine in which the strip is held by tenters mounted on chains, of which the spacing increases gradually.
When the system is started, the primary strip is introduced at low speed into the longitudinal drawing machine. When it is introduced, the drawing ratio is approximately 1, so that the strip retains its original thickness. It is accordingly too thick to be introduced directly into the tenters of the transverse drawing machine.
Under these conditions, the strip is first placed on an auxiliary winder disposed between the longitudinal drawing machine and the transverse drawing machine, then the longitudinal drawing ratio is gradually increased either to the final ratio or to an intermediate ratio, which results in reduction of strip thickness and increase of speed.
When the desired drawing ratio is reached, the strip is removed by hand and introduced into the tenters of the transverse drawing machine to be drawn transversely before being brought to a winder disposed downstream of the machine.
When these operations have been performed, the speed is gradually increased up to the desired production speed. However, such operations are complex and dangerous for the operators, and result in a substantial loss of material on fast high-output machines. In a modem system, the production rate is greater than 300 meters of strip per minute, while it is extremely difficult to manually introduce a strip into tenters at production rates greater than 150 meters per minute. The substantial difference between the two speed levels makes it necessary for there to be a speed run-up period which is lengthy, and during which film quality is unsatisfactory and not yet stabilized.